This invention relates to the monitoring of the integrity of containments particularly but not exclusively for nuclear plants.
In Canada, nuclear reactor containments may be single unit designs in which individual reactors are housed by their own independent containment, or multi-unit designs in which the containment provisions or portions thereof are shared by a number of reactors on the same site. While this application is primarily concerned with containment monitoring in the context of the single unit design, it is expected that the concepts presented will be universally applicable to any containment design, including containments for non-nuclear applications such as biological laboratories.
Most containments are equipped with ventilation systems and other auxiliary systems which penetrate the containment boundary. In the event of an accident, isolation valves on all of the major lines will automatically close to isolate the containment atmosphere from the outside environment. The isolation valves are built to rigid standards and tested frequently to ensure that closure speeds and leakage characteristics will preclude radionuclide releases in excess of allowable levels. Hence, known breaches are dealt with as a part of normal containment design.
However, impairments or inadvertent breaches of the containment envelope, such as seal failures or valves left in improper states, can occur during the life of the plant. These breaches are easily identified in integrated leakage rate tests, but as such tests involve reactor shutdown, containment isolation, and subsequent pressurization or depressurization to nonatmospheric levels, they are normally performed on an infrequent (years) basis. Hence, inadvertent breaches can remain undetected for significant periods of time.
The nuclear industry has long been interested in a means of monitoring containment integrity on a continuous basis, that is, while the reactor is operating normally. However, absolute success has been thwarted, in part, by the need to reliably distinguish inadvertent breaches from the legitimate breaches represented by system penetrations (such as the ventilation system). Consideration has been given to schemes such as natural resonance of the containment atmosphere, tracer decay approaches, and close monitoring of the containment pressure response signature to selected periodic pressure forcing functions.
From these investigations, the most promising approach appears to be to employ sensors and systems which automatically measure changes in the mass of air in containment, time-integrate any known air mass flow rates across containment boundaries and perform a mass balance to obtain the air mass leaked. However, as fluctuations in such measurements are typically too large to enable leakage to be calculated to the desired precision, filtering and statistical techniques must be employed to filter out random and time-dependent fluctuations. Current approaches cannot easily deal with non-random or systematic fluctuations in the measurements, such as pressure changes within the containment. As a result, sampling periods must be kept short or data measured during periods of varying containment conditions must be discarded.
Continuous monitoring of the mass in containment involves first determining the free volume of the containment and then continuously measuring the pressure, temperature and humidity throughout the containment, as well as the outside atmosphere. The presence of additional gas, if any, would require measurement also. The above parameters are sufficient to determine the air mass leakage across the containment boundary by a simple mass balance.
Trans-boundary flows present no problem in principle as long as they can be measured with sufficient precision to be included in the mass balance without obscuring the true leakage rate. The problem is that the leakage for nuclear containments is so small relative to the contained air mass and the integrated trans-boundary flows that instrument noise and drift as well as random and systematic fluctuations in the measured variables tend to mask the estimated leakage. To separate the random variations from the data, it is thus common practice to employ statistical techniques.
There are a number of methods for dealing with continuous mass measurements but only the so-called "mass point" method will be herein considered. Firstly, the governing mass balance equation (see LaFortune (1)) may be written as EQU y=(m.sub.in -m.sub.out)t+M.sub.acc (LR)t
where,
m.sub.in =known air mass flow rate into containment (kg/hr) PA0 m.sub.out =known air mass flow rate out of containment (kg/hr) PA0 M.sub.acc =air mass change in containment (kg) PA0 LR=air leakage rate (kg/hr) PA0 t=time (hr) PA0 .DELTA.P=pressure differential across the containment envelope PA0 k=leakage rate constant PA0 a.sub.1 and a.sub.2 are constants.
The "mass point" method consists of a linear regression of the contained mass over time and use of the slope of the regression line as the leak rate and the intercept as the initial contained mass. Reference 2 (Keogh) recommends this approach as the only method to be used. However, the weakness in this technique is an implicit assumption that the leakage rate is independent of pressure.
Reference 3 (Zakaib) also discusses this method and indicates that, in practice, some of the basic assumptions for linear regression are often violated, for example the assumption of normally distributed and independent random errors. Systematic errors due to actual physical variations in containment conditions (common during on-power testing) are thus incompatible with this approach and must be limited in magnitude.